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‹‹‹ Contents page for this issue     |     Winter 2012

Sweet miracles

By Susan Trulove

While American biofuels researchers and entrepreneurs are focused on liquid fuels, such as ethanol, Y.H. Percival Zhang is advocating and inventing new processes for creating hydrogen fuels from biomass.

U.S. Secretary of Energy Steven Chu questioned the future of hydrogen energy and hydrogen-powered fuel cells in a 2009 interview with Technology Review, saying "four miracles need to happen before hydrogen fuel cells can be practical… We need better ways to produce, distribute, and store hydrogen, and we need better, cheaper fuel cells."

Y.H. Percival Zhang is undaunted. The associate professor of biological systems engineering has already come up with a way to produce the highest yield of hydrogen from biomass sugar. One miracle. And he has an idea and successful results from proof-of-concept experiments for hydrogen storage and distribution. Miracles two and three are on the drawing board.

Enlisting enzymes to deliver renewable energy

Zhang grew up in Wuhan – a large industrial city in central China. Had he remained there, he might have pursued his youthful interest in physics, but Zhang's family moved to the smaller city of Wuxi when he was 14.

When he graduated from high school in 1989, his parents advised him to study biochemical engineering, an applied science, and biology, a future area of development for China. Zhang decided to study biochemical engineering at the relatively nearby East China University of Science and Technology in Shanghai.

"The whole university specialized in chemical engineering but I was able to pursue biology because of the national interest in the life sciences," Zhang explains. "I was interested in developing low-cost production processes for antibiotics and drugs because it was so important for China not to have to import pharmaceuticals."

While studying for his master of science degree, he was working on plant cell cultures with the aim of producing plant-based pharmaceuticals. "But when I decided to go to the United States to study for my Ph.D., I began to think how my education and interests could address other critical needs. An article in Science magazine by Lee Lynd at Dartmouth about renewable energy changed my research focus."

Lynd, a distinguished professor of engineering and an adjunct professor of biology, has received numerous awards for outstanding contributions to the field of biotechnology for fuels and chemicals. He is also chief scientific officer at Mascoma Corporation, formed to develop process technology for cost-effective conversion of plant biomass.

Zhang is persuasive and had earned a number of academic honors himself, even as a student. Lynd accepted Zhang as a Ph.D. student in biochemical and chemical engineering, and it changed the direction of both men's work. Zhang's research went from growing plant cells in a reactor to developing a means to break up plant cell walls in order to create an energy resource and other products.

He received his Ph.D. from Dartmouth in 2002 and stayed on for two more years as a postdoc and a year as a research scientist. During the years in Lynd's lab, Zhang's work with Lynd elucidated why the heat-loving, cellulose-eating bacterium – Clostridium thermocellum – can produce enough of the energy-carrying molecule, ATP, for synthesis of cellulase, the enzyme that breaks down cellulose. ATP is necessary for microbial growth and cellular production. Cellulase and ATP are needed to break apart cellulose into useful glucose polymers – a type of sugar molecule. Later, Zhang and Lynd found that by adding phosphate to the cellulose substrate for their workhorse bacterium, they could initiate a novel metabolic mechanism that would generate even more ATP. This discovery was published in the Proceedings of the National Academy of Sciences in 2005. As a result, one-step fermentation – integrating cellulase production, cellulose hydrolysis, and ethanol fermentation – is widely accepted as a low-cost platform for producing cellulosic ethanol.

Lynd notes that the achievement is a result of Zhang's development of a method for quantifying cellulase and cell production, followed by work to control cellulase synthesis in C. thermocellum. Zhang then undertook basic research to discover the biochemical relationship between the action of enzymes, specifically cellulase, and the physical features of cellulose. Lynd says Zhang attacked both projects despite Lynd's initial discouragement because of the difficulty they posed.

Zhang and Lynd's 2004 article about enzymatic hydrolysis of cellulose was the most-cited paper in Biotechnology and Bioengineering that year and was among the most downloaded papers seven years in a row (2004-10).

By 2005, Lynd's 10 patents included three with Zhang, who was ready to start his own career. "Virginia Tech was doing a biomaterial/bioprocessing 'cluster hire', so I was able to join a group in a growing program," says Zhang.

As part of the Virginia Tech bioenergy group, Zhang began to think about the roadblocks to biobased energy, such as the high cost of converting plants to fuel. His research became more applied. "Charlie Wyman was on my Ph.D. committee and I was very influenced by his emphasis on how important lignocellulose pretreatment is. It was clear that in the short term, pretreatment was the most urgent goal."

Zhang explains that producing ethanol from plant material involves three main steps: pretreatment to separate lignocellulose – the natural composite of lignin, hemicellulose, and cellulose that forms plant cell walls; breaking down the cellulose; and fermentation of the resulting sugars. There are potentially valuable byproducts from lignocellulose breakdown if a gentle enough process could be developed.

Zhang's undergraduate and master's research with plants and subsequent study of enzyme activity gave him unique insights, resulting in his development of a new, cost-effective pretreatment process. He introduced a cellulose solvent and organic solvent combination to replace the traditional high-heat, high-pressure process. The solvent solution can even be recycled. The weakened lignocellulose can be fractionated into four products: lignin, acetic acid, hemicellulose sugars, and amorphous cellulose. "While the sugars are the target, the lignin and acetic acid co-products can also generate income, making a biorefinery more profitable," says Zhang. "For instance, lignin has many industrial uses, from glue to polymer substitutes and carbon fiber."

He was generating public interest in his work despite the complexity of explaining how agricultural waste can be economically transformed into an energy resource. While presenting this enzyme work at the American Chemical Society meeting in 2006, Zhang had an epiphany.

Miracle one – hydrogen production

Why not develop an enzyme cocktail to convert sugars from biomass to hydrogen to power a fuel cell, and why not have a modest reaction occur onboard a vehicle or at a fuel cell site, Zhang asked.

Jonathan R. Mielenz, one of Zhang's research colleagues at Dartmouth, had become manager of the Bioprocessing Research User Facility at Oak Ridge National Laboratory (ORNL). Zhang was soon doing collaborative research with Mielenz using ORNL's special facilities to conduct hydrogen production experiments.

In 2007, Zhang and colleagues Barbara R. Evans and Mielenz of ORNL and Robert C. Hopkins and Michael W.W. Adams of the University of Georgia succeeded in completely converting starch and water into hydrogen using a combination of 13 enzymes never found together in nature.

Starch is used by plants for energy storage and is very stable until exposed to enzymes. Just add enzymes to a mixture of the starch and water and "the enzymes use the energy in the starch to break up water into carbon dioxide and hydrogen," Zhang says. The hydrogen is used by the fuel cell to create electricity. Water, a byproduct of that fuel cell process, will be recycled for the starch-water reactor. Experiments conducted at ORNL using off-the-shelf enzymes from bacteria, yeast, rabbit, Archaea microbes, and spinach confirmed that it all takes place at low temperature – about 86 degrees F – and at atmospheric pressure.

Publication of the research in the Public Library of Science open source journal (PLoS One) generated international media attention. Der Spiegel had fun pointing out that one of the enzymes comes from the stomach of a rabbit, so it was like putting a rabbit in your tank. (Rabbits have to digest greens for energy, after all.)

In 2009, the research team announced they had tweaked the enzyme cocktail and could convert cellulosic materials from nonfood sources to hydrogen gas pure enough to power a fuel cell by adding 14 enzymes, one co-enzyme, and water heated to about 90 degrees. "In addition to converting the chemical energy from the sugar, the process also converts the low-temperature thermal energy into high-quality hydrogen energy – like Prometheus stealing fire," says Zhang.

The researchers used cellulosic materials isolated from wood chips, but crop waste or switchgrass could also be used. It is not necessary to use food, such as corn, Zhang emphasizes. "It is exciting because using cellulose instead of starch expands the renewable resource for producing hydrogen to include biomass," Mielenz said at the time.

An even more important development was yet to come. No longer would enzymes have to be gathered from plants, bacteria, yeast, and bunny bellies. Zhang has figured out how to assemble the most active enzymes without the cellular machinery. That is, he has eliminated the cellular machinery required for reproduction, repair, and other nonenergy-producing cellular metabolism. Instead, his enzyme team performs only the sequence of catalytic actions for the highest yield of hydrogen from sugar and water. The result is three times the hydrogen production by the best hydrogen-producing microorganism. And, because the cascade enzymes are cell-free, there is no fragile cellular machinery and no left over, irrelevant cell mass.

The cocktail would be cheap, Zhang says. It can all be produced by the bacterium E. coli. No expensive coenzymes, ATP, or membrane proteins are needed. A costly substrate is unnecessary.

But wait; there's more. Chemical pretreatment to break up the biomass, such as with dilute sulfuric acid, requires detoxification to remove the toxic chemicals. The synthetic cell-free enzyme mixture works in the presence of the toxin-infused liquid biomass, meaning that the detoxification step is unnecessary, reducing the cost of producing biofuels.

Synthetic cell-free enzymes can also be used to produce other biofuels, but Zhang believes in hydrogen. "There is much higher energy efficiency for hydrogen fuel cell systems – 50 percent and theoretically as high as 83 percent, compared to gasoline-powered internal combustion engines, which are only 14 percent efficient on average. Also, hydrogen does not generate significant pollutants. Furthermore, hydrogen separation from sugar solution is the least costly among all biofuels. Even initial sugar concentration is not important for hydrogen production."

He foresees a future where stationary power stations switch to fuel cell-based systems and we are driving sugar-powered fuel cell cars.

Miracles 2 and 3 – hydrogen storage and distribution

Yes, Zhang is recommending putting a form of sugar in your vehicle's tank.

Hydrogen gas – a small, energetic molecule that reacts with many materials – is difficult to store and to transport. His solution is to produce reactive carbohydrates – that is, carbon-hydrogen-oxygen molecules – at a biorefinery using local biomass resources. That product, a carbohydrate, would be the hydrogen carrier. No complex infrastructure required. You could buy it from a grocery store or dry goods outlet.

With sugar, water, and enzymes in your tank, you have a fuel kit for a PEM (proton electrolyte membrane) fuel cell vehicle. An onboard battery provides the instant energy for starting the vehicle while the enzymes get to work on their sugary snack. The fuel cell will recharge the battery later from excess sugar energy.

According to Zhang, "Low-temperature PEM fuel cells are used primarily for transportation applications due to their fast startup time, high energy conversion efficiency, low operating temperature (below 180 F), and favorable power-to-weight ratio."

Zhang and Mielenz wrote in a review in the Jan. 28, 2011, issue of the journal Energies, "When polysaccharides and water are mixed, no reaction occurs ... When the enzyme cocktail is added, hydrogen and carbon dioxide are generated spontaneously…. Our research showed that the gas produced by (synthetic cell-free enzyme pathway biotransformation) contains 67 percent hydrogen and 33 percent carbon dioxide. Hydrogen and carbon dioxide can be separated by membrane technology (or the) mixture can be directly used by PEM fuel cells with approximately 1 percent loss in fuel cell efficiency." The efficiency statement is based on a study by Zhang's lab published in the journal Energy & Environmental Science in 2011.

Zhang wrote in a Perspective column in Energy & Environmental Sciences that the process provides a number of special features suitable for mobile PEM fuel cells: high energy efficiency as a result of extracting all the chemical energy stored in the substrate sugars and some of the low-temperature thermal energy from the fuel cell; high hydrogen storage density; mild reaction conditions, at the same range of those of PEM fuel cells; nearly no costs for product separation; clean products for PEM fuel cells and easy power system configuration; and simple and safe distribution and storage of solid sugars.

"Carbohydrates as a hydrogen carrier would meet the U.S. Department of Energy's ultimate target for useful energy based on the mass of the entire onboard system in a light-duty vehicle (7.5 percent hydrogen by weight or 2.5 kilowatt hour per kilogram)," Zhang says.

Stationary energy sites, such as large fuel cell stacks, can also take delivery of carbohydrate powder from local or distant biorefineries and generate hydrogen by using an enzyme cocktail, says Zhang. It is also possible that satellite hydrogen generation stations could produce hydrogen to refill hydrogen-fuel cell vehicles.

The use of renewable carbohydrate as a hydrogen storage carrier addresses the challenges associated with storage, safety, distribution, and infrastructure, Zhang and Mielenz conclude in the review.

What about miracle four – better fuel cells? It's not his field, but he believes most fuel cell problems, such as cost and lifetime, have been solved. "In the long term, improving energy utilization efficiency through hydrogen-fuel cell electricity systems will be vital for sustainable transportation," he says.

In the meantime, there are still a number of process engineering challenges to overcome to implement sugar-powered cars, says Zhang – such as warm-up of the onboard bioreformer where the sugar and water are converted to gas, shut-down of the bioreformer, temperature control for the coupled bioreformer and fuel cells, mixing and gas release control for the bioreformer, and re-generation of used enzymes in the bioreformer. "But such technical challenges can be solved based on available engineering know-how if the great potential is widely realized," he says.

Having realized the value of his unique education – and to make sure the next generation realizes the potential and develops the skills to create energy miracles – Zhang is taking on students at every level, including doing show-and-tell for grade-schoolers. He leads an undergraduate course called Unit Operations in Biological Systems to show students how concepts from their basic science courses come together in applications. He finds room in his lab for first-generation college students and students from underrepresented groups. At the graduate level, he has introduced an Enzyme Engineering course to the biological systems engineering program at Virginia Tech to teach students how to use enzyme sciences and technologies to solve biotechnology and biochemical engineering problems; and he is planning a graduate course on Energy, Water, Carbon, and Sustainability. "My goal is to teach the next generation of scientist and engineers. We need people with skills and passion to address key challenges in the sustainability revolution."

Persistence is almost Yi-Heng Zhang's first name. Yi-Heng means perseverance. When he came to America, he selected an American name with a similar sound. The old-fashioned, seldom-used name of Percival makes him easy to find using Google. It was also prescient. Like his namesake, Sir Percival, a knight of the Roundtable who sought the Holy Grail, Zhang wrote in the Perspective column, "We are calling for international R&D collaborations to pursue the holy grail of the carbohydrate hydrogen economy."

 

Percival Zhang, associate professor of biological systems engineering, discusses the conversion of biomass to energy with Geoff Moxley, a 21009 master's degree recipient who is now with Novozymes North American biomass research and development group. Photo by John McCormick.